Objects which electrically represent a capacitance are widely tested using pulsed-voltage generators. The pulsed-voltage generators used are, in general, designed using the Marx multiplier circuit, which has been known since 1924 and has a plurality of stages which can be charged, in the case of which each stage has, connected in series, a surge capacitance and a switching device, in particular a switching spark gap, a parallel resistor connected in parallel with the surge capacitance and the switching device and, connected in series with them, a series resistor, and two stages are connected to one another such that they can be charged connected in parallel and can be discharged connected in series.
When testing the capacitance of a unit under test, the unit under test itself as well as a capacitive pulsed-voltage divider are in general connected to the last stage of the pulsed-voltage generator circuit, which pulsed-voltage divider reduces the flash pulsed voltage produced while the stages are being discharged to levels which can be processed by the measurement and recording devices. The capacitances of the unit under test and of the pulsed-voltage divider together with the existing parasitic capacitances form the load capacitance of the test circuit, which comprises the pulsed-voltage generator, the pulsed-voltage divider, the unit under test and the connecting leads and, in addition, because of the physical extent of the overall test arrangement, has unavoidable inductance. The load capacitance together with this inductance forms an oscillatory circuit, which is damped by the series resistors of the various stages which can be charged, the series resistors being also called end, front or damping resistors. In addition to damping, the series resistors are also used for adjusting the rise time of the voltage pulses to be produced. In order to achieve a desired rise time, the series resistors must be chosen to be smaller the greater the load capacitance.
The pulsed-voltage generators are normally used to produce a standard flash pulsed voltage of 1.2/50 in accordance with IEC 60-1 (1989), that is to say voltage pulses with a rise time of 1.2 .mu.s.+-.30% and a half-value fall time of 50 .mu.s.+-.20%. In order to comply with the specified rise time, the series resistors must be chosen to be so small for large load capacitances, that the flash pulsed voltage produced is a damped oscillation. Standard IEC 60-1 allows a maximum overshoot of 5% over the non-oscillating voltage profile. It follows from this that the theoretical maximum load capacitance, called the limit load capacitance from now on, of a pulsed-voltage generator results when the series resistors limit the overshoot to just 5% and the rise time reaches the upper tolerance limit of 1.56 .mu.s. Any increase in the series resistors would reduce the overshoot but at the same time increase the rise time, while any reduction in the series resistors would shorten the rise time, but would increase the overshoot.
In order to increase the limit load capacitance of the Marx multiplier circuit, attempts have already been made to use a suitable arrangement of stages which can be charged to design the pulsed-voltage generator to have as little inductance as possible. For example, known items include a meandering arrangement of stages which can be charged, splitting the current in the generator into two opposite current paths, or arranging all the components to be physically very close. With a conventional open configuration, a low-inductance design of the pulsed-voltage generator can, however, influence only a portion of the total inductance, since the size of the outer loop, comprising the unit under test and the connecting lead, is governed by the separations required to cope with the voltage and the geometric size of the unit under test. Improvements here are now possible only to a minor extent.
However, from the user's point of view, there is a requirement for limit load capacitances which are higher than those of the known Marx multiplier circuits, for example for testing SF.sub.6 -insulated system parts. For transformer testing as well, the pulsed-voltage generator is often loaded with wound capacitances, which no longer allow the standard rise time.
In CH-A-376 999 a pulsed-voltage generator circuit which has a plurality of stages which can be charged is described, in which overvoltages which arise in the interior of the pulsed-voltage generator during the striking of the pulsed-voltage generator are reduced by additionally installed capacitances. The overshoot of the pulse fronts of the flash pulsed voltage across the load capacitance cannot be reduced by these additional capacitances alone.
From CH-A-238 586 a pulsed-voltage generator circuit is known which has a single stage which can be charged and has a surge capacitance and a switching spark gap, and which has an additional circuit element having connected in series a spark gap, a resistor and a capacitance. This additional circuit element serves for shortening the rise time of the produced pulsed voltage and does not suit for reducing the overshoot of the pulse fronts of a flash pulsed voltage across the load capacitance.
The invention is thus based on the object of providing a pulsed-voltage generator circuit of the type mentioned initially, by means of which a higher limit load capacitance can be achieved than with comparable, known circuits.
This object is achieved by the pulsed-voltage generator circuit according to the invention. An additional circuit element according to the invention, is a means by which the limit load capacitance of a pulsed-voltage generator circuit of the above-mentioned type can be increased.